Haggai (חגי, Standard Hebrew and Tiberian Hebrew Ḥaggay) was one of the twelve so-called minor prophets and the author of the Book of Haggai. He was the first of three prophets (with Zechariah, his contemporary, and Malachi, who lived about one hundred years later), whose ministry belonged to the period of Jewish history which began after the return from captivity in Babylon.
Scarcely anything is known of his personal history. He may have been one of the captives taken to Babylon by Nebuchadnezzar. He began his ministry about sixteen years after the return of the Jews to Palestine. The work of rebuilding the temple had been put a stop to through the intrigues of the Samaritans. After having been suspended for fifteen years, the work was resumed through the efforts of Haggai and Zechariah (Ezra 6:14). They exhorted the people, which roused them from their lethargy, and induced them to take advantage of a change in the policy of the Persian government under Darius the Great.
Haggai's prophecies have been characterized by the following: "There is a ponderous and simple dignity in the emphatic reiteration addressed alike to every class of the community, prince, priest, and people, 'Be strong, be strong, be strong' (2:4). 'Cleave, stick fast, to the work you have to do;' or again, 'Consider your ways, consider, consider, consider' (1:5, 7; 2:15, 18). It is the Hebrew phrase for the endeavour, characteristic of the gifted seers of all times, to compel their hearers to turn the inside of their hearts outwards to their own view, to take the mask from off their consciences, to 'see life steadily, and to see it wholly.'" (Stanley Jewish Church)
Initial text from Easton's Bible Dictionary, 1897 -- Please update as needed